High-frequency direction finder



6 1947. c. F. EDWARDS ETAL 2,419,946

HIGH FREQUENCY DIRECTION FINDER Filed Feb. 17, 1942 A TTORNE V PatentedMay 6, .1947

i UNITED STATES PATENT OFFICE HIGH-FREQUENCY BIRECTION FINDERApplication February 17, 1942, Serial No. 431,194

(Cl. Z50-11) 11 Claims. 1

This invention relates to direction finders and more particularly tohigh frequency and ultrahigh frequency direction finders.

As is known, high frequency direction finders are generally divided intotwo groups, namely, the phase-angle comparison type and the amplitudecomparison type. The copending application Serial No. 741,372, H. T.Budenbom, led August 25, 1939, and Patent 2,234,654, W. Runge, March 1l,1941, illustrate the phase-angie comparison type and United StatesPatent 2,234,587, H. T. Budenbom, March 11, 1941, and 2,213,874, C. F.A. Wagstaife, September 3, 1940, illustrate the amplitude comparisontype. Briefly, in the phase-angle comparison type', a pair of currentshaving a phase-angle relation representing wave components received byspaced non-directional antennas are supplied to a phase indicator, thephase-angle relation of the currents being dependent upon andrepresentative of the path or line-direction of the wave and the currentamplitudes being equal or unequal. In the amplitude comparison system, apair of currents received on antennas having dissimilar directivecharacteristics, or angularly related similar characteristics, aresupplied to the indicator, the currents having an amplitude relation andan instantaneous phase polarity relation representing the waveline-direction. In general, assuming cathode ray tube indicators areused in both types of systems, the shape (circular, elliptical orlinear), and not the orientation, of the pattern obtained in the phasecomparison system indicates the wave direction provided, of course, thatthe currents supplied to the indicator, hereinafter termed indicatorcurrents, have a phase-angle relation corresponding to that of thereceived energies. On the other hand, in the conventional amplitudedirection finding system, the orientation of the longest axis of thetrace, rather than the shape or contour of the trace, indicates the wavedirection, provided the separate receiving channels have equal gains andfunction to preserve the instantaneous polarity relation of the receivedenergies. While a linear trace is theoretically obtainable in theamplitude system with the phase-angle shifts in the two receivingchannels equal, such a trace has not always been obtained in the pastbecause of the difficulty of constructing receiving channels havingequal phase-angle shifts, or phase shifts differing only a few degrees,at the received high frequency or at the several high frequencies in aband of frequencies. In accordance with the present invention, therelative phase shift requirement for this type of direction finder ischanged or reduced from one permitting only a few degrees shift at ahigh frequency to one permitting a few degrees shift at an audiofrequency. The shift at an audio frequency corresponds to a shift ofseveral thousand degrees at the high frequency, whereby a linear traceis more easily obtained.

In addition, `in the amplitude system, an exact correlation between theamplitude relation of the indicator currents and the amplitude relationof the received energies is required for successful operation. In thephase-angle comparison system, the amplitudes of the indicator currentsmay, as stated above, be unequal, but Vthese amplitudes are preferablymade equal in order that, with the antenna in a so-called olf-bearingposition, the degree of deviation from the in-bearing position may beaccurately gauged. Inasmuch as it is not feasible and is in factexceedingly difficult, if not for all practical purposes impossible, tomatch exactly the gain characteristics of the two receiving channels,each channel including a plurality of tubes, the volume controlarrangements heretofore employed in both the phase-angle comparison andthe amplitude comparison systems, have in general failed to secureequiamplitude indicator currents. In accordance with the presentinvention the gains of the two receiving channels are maintained equalduring the entire direction determination operation.

It is one object to improve vthe operation and accuracy of highfrequency direction finders.`

It is another object of this invention to provide an accurate, directand instantaneous indication of the line-direction of the incoming wave.

It is another object of this invention to render the gains in the tworeceiving channels of a phase-angle comparison direction finder and anamplitude comparison direction finder equal and independent of both thearrival direction and the fading of the wave incoming from the distanttransmitter.

It is a further object of this invention to obtain a linear trace in anamplitude direction finding system comprising a cathode ray tubeindicator and receiving channels having unequal phase angle shifts.

It is still another object of this invention to obtain a correctindication in phase-angle comparison and amplitude comparison directionfinding systems utilizing receiving channels having equal delaycharacteristics but not necessarily equal phase-angle shifts.

In accordance with one embodiment of the invention, a local injectiontransmitter is associated with an amplitude comparison direction findercomprising e, pair of crossed Adcock antennas connected through separatereceiving channels to an indicator. The local transmitter is adjusted totransmit a frequency differing by an audio frequency from that of theincoming wave and its antenna is located on a line making 45 degreeswith the quadrature axes of the Adcock antennas so that equiphase-angle,equiamplitude wave components are supplied to the Adcock antennas. Thereceiving channels each include a radio frequency amplier, a iirstdetector and an intermediate frequency amplifier. A second detector isconnected to the output of each channel for detecting a current havingan audio frequency equal to the diiierence between the injection wavefrequency and the incoming wave frequency. Preferably, a beat frequencyoscillator common to both channels is connected to the first detectors,although separate beat frequency oscillators may be used. Thefrequencyphase angle characteristics Vof the two receiving channels,over a frequency band having a width equal to the magnitude of thedetected frequency, are similar, whereby the phase polarity relation andthe phase angle relation of the antenna currents are reproduced in thedetected currents, and a purely linear trace representing thelinedirection of the incoming wave is obtained on the indicator. Thefrequency-phase angle characteristic represents the rate of phase angleshift with frequency change and hereinafter will be designated thedelayf In addition, the amplitude of the injection wave is considerablygreater, for example, 26 decibels, than that of the incoming wave, sothat in both receiving channels the volume is continuously controlledprimarily in accordance With only the injection wave intensity, wherebythe gains in the two receiving channels are maintained equal andindependent of both the `arrival direction and the fading of the waveincoming from the distant transmitter` The gains in the two receivingchannels are, after initial adjustment of the individual volume controlcircuits, maintained equal regardless of whether or not the volumecontrol charactertiscs of the` two receiving channels are the same,although if they are not identical, their slopes should be the same.

The invention will be more fully understood from a perusal of thefollowing specication taken in conjunction with the drawings on whichlike reference characters denote elements of similar function, and onwhich:

Fig. 1 illustrates one embodiment of the invention; Fig. 2 illustrates aplan View of the antenna system used in Fig. l; and Fig. 3 is a curveused in explaining one feature of the invention.

Referring to Figs. 1 and 2, reference numerals I 2, 3 and 4 indicatefour vertical large capacity type antennas positioned at the corners ofa square for receiving a wave incoming from a distant transmitter andhaving a frequency F1. Preferably, each antenna element comprises awooden supporting structure completely covered by sheet copper. Whileelements of square cross section are illustrated, the cross section maybe other than square as, for example, circular. Antennas I and 2constitute one so-called Adcock directional antenna array and antennas 3and 4 constitute another Adcock antenna array 6, the arrays 5 and 6being positioned on the axes 'I and 8, respectively, and having their`directions of maximum radio action perpendicularly related. Numeral 9designates two buried coaxially shielded line conductors of equal lengthconnecting antennas I and 2 of array 5 to the input terminals II) of thebalanced-to-unbalanced transformer I I, and reference numeral I2 denotestwo similar line conductors connecting antennas 3 and 4 of array 6 tothe terminals II) of balanced-to-unbalanced transformer I3. Eachtransformer assembly comprises a primary winding I4, shunt condenser I5,split secondary winding I6 and two condensers I'I connected in series.Numerals i8 and I9 designate unbalanced ccaxial lines connecting,respectively, the output terminals of transformers II and I3 to theinput terminals 20 of the radio receivers 2I and 22. Each of the radioreceivers 2l and 22 comprises an impedance attenuation pad 23, radiofrequency amplifier 2d, rst detector 25, intermediate frequencyamplifier 26, a second or low frequency linear detector 2l for obtainingan audio frequency voltage from the intermediate frequency output and anaudio amplifier 28 having a manual gain control. Numeral 29 designatesan adjustable beat frequency oscillator common to receivers 2| and 22and connected to the input terminals of the first detectors 25.

In accordance with well-known circuit arrangements, in each receiver a.portion of the intermediate frequency output is supplied through anadjustable automatic volume control circuit 30 to amplifiers 24 and 26as a grid bias. The gain control circuits 3l) each comprise a rectifierand a smoothing circuit having a long-time constant I for obtaining adirect current control voltage from the intermediate frequency output.Gain control circuits 30, are, by special design, made sharplyresponsive, that is, extremely stiiif so that the receiver gains are thesame, substantially, over a large intensity range fol` the wave incomingfrom the injection antenna 39. Pads 23 function to prevent overloadingof the receivers when exceptionally strong incoming Waves are received.The output terminals of receivers 2I and 22 are connected, respectively,to the plates 3| and 32 of the cathode ray tube indicator 33. Consideredfrom a slightly different standpoint, the system of Fig. 1 comprises twodistinct radio frequency-intermediate frequency receiving channels A andB. Channel A comprises the directive antenna array 5, coaxially shieldedconductors 9, transformer II, coaxial line I 8 and the high frequencyportion of receiver 2I comprising pad 23, radio frequency amplifier 24,first detector 25 and the intermediate frequency amplifier 26; andchannel B comprises directive antenna array 6, coaxially shieldedconductors I2, transformer I3, coaxial line I9 and the high frequencyportion of the receiver 22 corresponding to the above-described portionof receiver 2|. As explained below, the two receiving channels haveequal gains and equal delay characteristics.

Numeral Sil designates a variable local source of radio frequency wavesadjusted to supply a frequency F2 diiiering from the incoming frequencyF1 by an audio frequency amount which is preferably in the order of to500 cycles, but which may be higher and is limited only by the bandwidth of the receivers. The oscillator 34 is connected by line 35through a modulator-amplifier 3E and the adjustable attenuator 3l to thecoaxial line 3d and the non-directional vertical anten- 99 hereinaftercalled the injection antenna."

. If desired, antenna 39 may be directional, the only requirement beingthat the intensities of the wave `components; emitted by the antenna 39and receivcd by antenna arrays and G are, as received, equal. Theantenna 39 is symmetrically located relative to the angularly relatedreference directions or antenna axes 'i and 8 and, preferably, islocated on a line making an angle of 45 degrees with each of theabove-mentioned axes and at a point suiiiciently close to the arrays 5and 6 to obviate ground factor differences in the paths extendingbetween antenna 39 and the four vertical antenna elements i, `2, and 4.Numeral i6 denotes an auxiliary audio frequency oscillator forgenerating a wave f=F1F2. As explained below, oscillator may beconnected by means of switch M to the modulator-amplifier 36, in theabsence of the incoming wave Fi, for the purpose of calibrating thereceivers 2l and 22.

In operation, the beat oscillator 29 is varied until receivers 2! and 22are adjusted to receive a wave having an unknown direction as, forexample, that indicated by arrow 2, and a frequency F1, which may be anyhigh frequency. The incoming wave establishes in the two verticalantennas e-f each array voltages having a phase angle relation dependentupon the azimuthal angle between the wave line-direction 132 and therespective array axis i or 3. Since, in each array, the two verticalantennas are connected in series through the primary winding i4 of theassociated transformer il or [3, a radio frequency current having anamplitude corresponding to the above-mentioned angle is established bythe absorbed voltages in the associated primary winding i4. Hence thecurrents in the primary windings of transformers H and i3 have anamplitude relation which is represen tative of the size of the anglebetween the wave line-direction and the axis or reference direction, forexample, direction The angle between the wave direction and axis 3 isthe compleinent of the angle just mentioned. Also these currents have aninstantaneous similar or opposite phase polarity relation dependent uponwhich of the two pairs of oppositely facing angles (C, C or D, D)includes the line-direction 2. More particularly, referring to Fig. 2and assuming the four angles made by axis 8 and each of possible Wavecompass point directions 42, ll3, 44 and i5 are equal and that thepolarities have the same sense for direction 412, the polarities are ofthe same sense for direction i3 but of op posite sense for each ofdirections M and 35. The currents mentioned above do not represent thesense or point-direction of the incoming wave. In this connection theterm phase-polarity as used herein should be sharply distinguished fromthe term phase-angle also used herein. By phase-polarity is meant thesense or direction of the energy, voltage of current, relative to aphysical or geometrical point to or from which energy iiows.Phase-polarity is ordinarily denoted by the mathematical sign plus orminus; is not indicative of a quantity and is not measured in degrees.By phase-angle is meant the electrical degrees measured on a time axisand having a value cos 2W ft, where t is the distance on the axis from areference point on said axis. It may be noted by way of furtherexplanation, that a phase-angle change of 181 degrees in a currentpassing through a particular device reverses the sense or phase-polarityof the current, whereas with a phaseangle change of 179 degrees noreversal of phase-polarity occurs.

Simultaneously with the reception of frequen-A cy F1 on arrays 5 and 6,the injection oscillator 34 supplies a wave F2 to the local injectionantenna 39 through modulator-amplifier 36, adjustable attenuator 3l' andline 38, the switch il being normally opened and the audio oscillator i0disconnected from the amplifier 35. The injection system comprising theantenna 3Q emits a wave having a direction E5 and functions to establishin arrays 5 and 6 and the primary windings i4 of the respectivetransformers Il and i3, equiamplitude equiphase-angle currents.Normally, attenuator 31 functions to render the wave F2 from antenna 39,twenty-six decibels stronger (approximately 20 to- 1 ratio) than theincoming frequency Fi. If necessary, any wide deviation in the aboveratio may be corrected by adjusting attenuator 3T- After passing throughtransformers Il and i3 the waves F1 and F2 are conveyed by lines I8 andI9, which are preferably of equal length, to the receivers 2i and 22. Asexplained below, in each receiver the waves are attenuated by pads 23 toa given value, amplified by the radio frequency amplifier 2d and thencombined in the first detector 25 with a wave of frequency F3 from thecommon beat oscillator 29 The intermediate frequency waves `I1-F3=F4 andF2Fa=Fs are conveyed to amplifier 2E. One portion of the output ofintermediate frequency output of amplier 25 is supplied to detector 21and another portion is supplied to the rectifier in the gain controlcircuit Sil. The linear detector 2 functions to detect a current havinga low frequency Fi-F :f and an amplitude variation corresponding to thatof F1 and independent of F2. In this respect each receiver simulates asingle side-band receiver for receiving a wave with a low percentagemodulation and comprising a linear detector, wherein the audio output isproportional to the amplitude of the side band (incoming wave) andsubstantially independent of the carrier (injection wave). The twodetected audio frequency currents f in receivers 2| and 22, hereinafterdenoted, respectively, ,f1 and f2 are then amplified by audio amplifiers28 and impressed, respectively, on plates 3! and 32 of the cathode raytube indicator 33. A linear trace, the orientation of which relative tothe reference point or line on the indicator scale is indicative of thewave direction, is obtained on tube 33, As

will now be explained, the linear trace is obtained, although thereceiving channels A and B may have unequal phase-shift characteristics,

As pointed out previously, in the amplitude comparison systemsheretofore employed, a highly desirable ideal linear cathode ray tubetrace has not been readily obtained inasmuch as the receivingarrangements utilized were such that, to secure a linear trace, thephase-angle shifts in the receivers must be equal, as shown by curve ilin Fig. 3; and it has not been feasible tol construct receivers havingequal phase-angle characteristics. Any deviation in the phase-anglerelation between the indicator currents from a relation corresponding tothat of the antenna currents causes the trace to split open and assume aquasi-elliptical shape. In accordance with the invention, the receivingchannels A and B each including an antenna array and other radiofrequency apparatus, as stated above, are designed to have equal delaycharacteristics. Thus, referring to Fig. 2, at each of the two radiofrequency waves F1 and F2 the phase-'angle shifts in the receivingchannels A and B are substantially different. For example, for frequencyFi aci-amas the 4,phase-angle shifts `in the receiving channels A andBmay be 60 and 63 degrees, respectively, and at frequency F2 they may be,respectively, 80 and 83 degrees. The only requirement is that therelative phase shifts over a frequency band including F1 and F2 areequal. In other words, the delays or slopes of the curves d8 and 49,Fig. 3, should be the same, More accurately, the relative phase-angleshifts for the two particular frequencies F1 and F2 should be equal,regardless of the relative phase-angle shift for frequencies betweenthese two frequencies. The delay characteristics may, of course, benon-linear and equal as shown, for example, by the dotted curved linesiiand i in Fig, 3. With equal delays in the two receivers the detectedcurrents have phase polarities, and also phase-angles, corresponding tothe phase-angles of the antenna currents. Hence, in accordance with thisinvention and by reason of the introduction of a-n injection frequencyF2 differing by an audio frequency from the incoming frequency F1, thephase-angle requirements for the radio frequency and intermediatefrequency portions of the receivers included in amplitudecomparisonsystems are rendered less strict than heretofore; and equal phase-angleshifts in the detected currents may be obtained merely by usingreceivers having equal delays. It is a relatively simple matter todesign and construct radio frequency and intermediate frequency elementshaving equal delays and unrelated phase-angle shifts. The audioamplifiers 28 should have equal phase shifts since only a singlefrequency is conveyed by each of these devices. Audio frequencyamplifiers with equal phase shifts are relatively easy to constructsince the wave-length is eX- ceedingly long and the percentage change inphase-angle is small during the amplification process.

In connection with the above, sharp distinction should be made betweenthe use of the injection oscillator and the associated equal delayreceiving channels, as used in applicants system, and the phasingoscillator and linear phase shift receivers used in phase comparisonsystems as, for example, the system disclosed in the H. T. Budenbomapplication Serial No. 741,372, mentioned above. In applicant'samplitude system, the orientation of the cathode ray tube traceindicates the wave direction; and the orientation is dependent primarilyon the amplitude relation of the antenna currents. The directionindication is produced on the tube indicatoreven if the phase-anglebetween the detected currents is not actually correlated to thephase-angle between the antenna currents, provided that the deviation isnot greater than 179 degrees, that is, not great enough to reverse thephase-polarity of the currents. Stated differently, in the amplitudecomparison system, the phase-angle factor, when not large, relates tothe sharpness of the indication, and not tc its production. On the otherhand, in the above-mentioned phase comparison system, any disagreementwhatsoever between the phase-angle of the detected currents and thephase-angle of the absorbed components of the incoming wave materiallyaffects the shape of the trace and hence prevents the establishment of,or utterly destroys, the directional indication. With the antenna arrayaxis in the inbearing position, in the phase system, a line indicationis obtained, regardless of the amplitude relation of the currents; andthe amplitude dissimilarity merely affects the line orientation which is.not indicativeof wavedirecton. Equal amplitudes 'are sometimes utilizedin the phase system in order to secure, in the off-bearing condition, asharp trace representative of the deviation of the system from theirl-bearing position. Hence, in general, in the phase-angle comparisonsystem, the amplitude factor relates to the sharpness of the indicationobtained, and the correlation of the phase-angles of the detected andabsorbed energies is a factor upon which the production, rather than thesharpness, of a trace representing the wave direction is dependent.

Referring again to Fig. 1 and considering the portion of theintermediate frequency output, F4 and F5 of amplifier 25 supplied ineach receiver to the gain control circuit 38, the rectifier functions toproduce a rectified control voltage having an intensity which, becauseof the large difference in intensity of waves F1 and F2, is a functionprimarily of the intensity of the injection wave F2. The largedifference in intensity of waves F1 and F2 and the smoothing circuitfunctions to eliminate the alternating current ripple or variation. Fora given input intensity of wave F2 the circuits 3D are adjusted to givethe same gains. Inasmuch as the gain control characteristics, that is,the gain-grid bias curves of the two receivers may differ, and thereforethe rectified voltages utilized to secure equal gains for the same inputF2 may differ, the slopes of the above-mentioned characteristics shouldbe the same over the complete input level range of the receivers sothat, for slight intensity variation in the substantially constantintensity of the injection ywave F2, the gains or changes in gain arethe same. In other words, the receivers must respond equally or trackeach other upon changes in input intensity. The gain control requirementis analogous to the delay requirement illustrated by Fig. 3. If desired,prior to the reception of incomingr frequency F1, the two receivers maybe calibrated as follows: The audio frequency oscillator Sil generatinga frequency f is connected by switch 4! to the modulator-amplifier 3Band, in each receiver 2l and 22, a detected frequency f is obtained fromthe locally transmitted wave Fai-f. The gains of the audio amplifiers 28are adjusted so that their outputs are equal. After the preliminaryadjustment, the audio oscillator Il is disconnected frommodulator-amplier 35, Thus, in accordance with the invention, the gainsof the two receivers are automatically maintained equal during thesubsequent reception of the incoming wave F1, without destroying theamplitude relation representing the wave direction, by supplying theunmodulated injection wave F2 to the two receivers and obtaining fromwaves F1 and F2 the detected frequencies f1 and f2 andy the rectifiedgain control current. Moreover, the gains of the receivers aremaintained constant and independent of both the arrival direction andthe fading of the incoming wave F1.

Although the invention has been described in connection with a certainembodiment it should be understood that it is not to be limited to thisvparticular embodiment inasmuch as the invention may be successfullypracticed with other apparatus. 1n particular, the invention isapplicable to other direction finding systems of either the phase-cramplitude comparison type and is espe-- cially applicable to amplitudecomparison sys. tems using other antenna arrays as, for example, thearrays disclosed in Patent 2,119,667, E. J. Sterba, June'?, 1938, andcomprising a non-directive antenna and -a vdirective antenna or two`horizontally polarized "having different vertical planecharacteristics.

What is claimed is:

antenna-ground systems 1. In combination with a-sys'tem for determin- Aing the direction of an incoming wave comprising a pair of receivingchannels each having a separate automatic gain regulator controlled bythe received energy, means for maintaining the gains in said channelsequal and substantially independent of the arrival direction and fadingof said incoming wave comprising a` source of radio energy connectedthrough pathsof equal continuously supplying to said channels componentsof a control wave having a large magnitude relative to said incomingsignal.

3. In combination with a system for determining the direction of anincoming wave, said system comprising a pair of separate receivingchannels connected to an indicator and each having an antenna and aseparate gain regula-` tor controlled by the received energies, meansfor maintaining the receiver gains substantially equal comprising alocal transmitter including an antenna symmetrically located relative tosaid antennas for supplying to said channels a control wave having alarge magnitude compared to that of the incoming wave.

4. In combination with an amplitude-comparison direction findercomprising a pair of separate receiving channels and a common indicator,said receiving channels each including a separate automatic volumecontrol circuit controlled by the received waves, means for maintainingthe gains of said channels equal during the direction findingdetermination comprising a, local transmitter including an antenna forcontinuously supplying to said channels equiamplitude and equiphaseenergy components each having a `large intensity relative to that of theincoming r wave.

5. In combination, a pair of receiving channels each channel comprisinga directive antenna connected to an amplifier, separate detectorsconnected to the outputs of said amplifiers, a. common cathode ray tubeindicator connected to said detectors, and means for obtaining lowfrequency indicator currents representing the intensities and therelative phase polarity of the energies received by said antennas, saidmeans comprising a local transmitter including an antenna for supplyingto said channels equiphase currents having a high frequency differingfrom the high frequency of the incoming wave by an amount equal to saidlow frequency, and said amplifiers having equal delay characteristicsover a frequency band including said high frequencies.

6. In an amplitude-comparison direction nding system, a pair ofreceiving circuits for obtaining currents having an amplitude relationrepresenting the incoming wave direction and each comprising an antennaconnected through a separate radio frequency amplifier and a separatedetector to a common cathode ray tube inanemie `dicator, means forsupplying to said antennas equiphase components of a Ywave having afrequency differing from that of the incoming Wave, said amplifiershaving substantially eqal relative phase-angle shifts forfrequencieshaving a difference equal to the difference between the frequencies ofsaid waves.

7. In combination, a pair of directive antennas for receiving a desiredwave and positioned for different directions of radio action, saidantennas being connected through separate amplifiers to separate10W-frequency detectors, means for supplying to said antennas andassociated ampliers a local wave having a frequency differing from `thatof the desired wave, said amplifiers Vhaving equal delay characteristicsover a frequency band having a width at least as great as the magnitudeof Said frequencyV difference,j and means connected to said detectorsfor comparing the intensities of their outputs.

8. In a radio system for determining the direction of a desired incomingwave, a plurality of separate transmission channels each including adirective receiving antenna and at least one amplier, said antennashaving angularly related directions of maximum radio action in a givenplane, means for supplying to said antennas equiphase equiamplitudecomponents of a local wave having a frequency differing from that oftheincoming wave, said channels having similar frequency-phase anglecharacteristics over a frequency band having a width at least as greatas the magnitude of the frequency diiference between said incoming andlocal Waves, detector means connected to said channels for obtainingfrom said incoming and local waves a pair of currents each having afrequency equal to the frequency difference between said Waves and acathode ray tube connected to said detectors for comparing theintensities of said currents.

9. In combination, a pair of directive receiving antenna arraysconnected through separate receivers to an indicator, each receivercomprising a radio frequency amplifier, an intermediate frequencyamplifier and a low frequency detector, an indicator connected to saiddetectors, said radio frequency amplifiers and intermediate frequencyamplifiers having similar delay characteristics, means for supplying tothe input terminals of said radio frequency ampliers a wave having afrequency F2 equal to Fiif where F1 is the frequency of the incomingdistant wave and f is an audio frequency corresponding to that of thedetected currents.

10. In an amplitude-comparison direction finder comprising two receivingantenna arrays for obtaining from an incoming wave a pair of radiofrequency currents having an amplitude relation representing the anglesincluded between the incoming wave direction and two predeterminedangularly related directions coinciding with the axes of said arrays,means for obtaining from said radio frequency currents a pair ofcurrents having a given low frequency and an amplitude relation equal tothe first-mentioned relation, said last-mentioned means comprising meansfor supplying to said antennas equiamplitude and equiphase radiofrequency waves having a frequency differing from that of said incomingwave by an amount equal lto said low frequency, separate amplifyingchannels connecting said antennas to separate detecting means and havingsimilar delay characteristics over a frequency band having a width atleast as great as the magnitude of said low frequencyI and means forcomi paring the intensities of the outputs of said detectors wherebysaid angles may be determined.

11. In combination, a pair of radio receiving channels each comprisingseparate antennaarrays'for obtaining from an incoming wave a pair ofradio frequency currents having an amplitude relation representing themagnitudes of the angles included in a given plane between thelinedirection of'said wave and each of two predetermined angularlyrelated reference line-directions coinciding with the axes of saidarrays, said currents having similar or opposite instantaneous phasepolarities depending upon which pair of oppositely facing angles formedby said reference directions include the wave line direction, means forobtaining from said currents a pair of low frequency currents having anamplitude relation and a phase polarity relation corresponding to thoseof said rst-mentioned currents, said means comprising an oscillator forsupplying to corresponding points in said channels equiamplitudeequiphase components of a wave having a frequency diifering from saidradio frequency by an amount equal to said low frequency and anintensity in the order of twentysix decibels greater than that of theincoming wave, at least one amplier included in each of said channels,said amplifiers having similar frequency-phase angle characteristicsover a frequency band at least as wide as the magnitude of Vsaid low`frequency, a volume control circuit in each channel for equallyregulating the gain of said channel ampliiers substantially inaccordance with only lthe Yindividual intensity of saidcomponentsjseparate detectors connected to the output 'of saidychannels, an indicator connected to said detectors for comparing theintensities of the detected current and effectively ascertaining theirphase polarity relation, whereby the magnitude of the first-mentionedangles and the orientation of the wave line-direction relative to theaforementioned oppositely facing angles are determined.

CHARLES F. EDWARDS.

KARL G. JANSKY.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,182,950 Steinhoif Dec. 12, 19392,170,835 Simon Aug. 29, 1939 2,226,366 Braden Dec. 24, 1940 2,262,931Guanella Nov. 18, 1941 2,083,495 Black et al June 8, 1937

